Radar is a measurement system used to detect and locate objects, and to determine their approaching or receding speeds. It must transmit a particular form of radio frequency (RF) signals in order to achieve one of these specific goals. Many types of radars are in use at the present. In general, they can be divided into two groups: continuous wave (CW) and pulsed.
Pulsed radars emit short RF signals in the form of pulses. These pulses are transmitted to and reflected by objects. From reflected pulses, one can detect the existence and locate the positions of these objects, which cause the reflection of RF signals. CW radars emit RF signals in the form of continuous waves, which have well defined RF frequencies. As the waves reflected by the objects in motions, the well defined frequencies are altered. The alterations are often referred to as Doppler shifts. The approaching or receding velocities of these objects with respect to the transmitting radars can be calculated from the Doppler shifts.
A pulsed RF wave does not have a well defined frequency. The spread in frequency is directly related to its pulse duration. The shorter is the pulse duration, the larger is the frequency spread, and the better in locating objects. A 100 nanoseconds pulse, which yields the target position with uncertainty of 500 feet, has a spread more than 10 MHz. The Doppler shifts for usual objects of interest are below several hundred KHz. The frequency spread makes a single radar pulse useless in determining the motions of objects. Multiple radar pulses are needed to reveal target motions; they are often referred to as moving target indication (MTI) and pulse Doppler radars. The major difference of these two pulsed radars are their pulse repetition rates. A MTI radar has a lower pulse repetition rate than that of a pulse Doppler radar. The MTI radar yields a clear unambiguous range determination, but the motion indication is poor and often ambiguous. The pulse Doppler radar with its high pulse repetition rate yields a clear unambiguous Doppler determination, but the range indication is poor and ambiguous. Both MTI and pulse Doppler radars require interpulse coherence among transmitted RF pulses which is an stringent requirement.
CW radars do not provide range information in locating positions of objects. One method to overcome the restriction is through the modulation of their continuous RF waves. An implementation of amplitude modulation changes a CW radar to a pulse radar. Another method is through the frequency modulation of their RF carriers. The ranging capability of frequency modulated radars depends on the frequency excursion. A greater frequency excursion leads to a better range resolution, but to a poorer capability in the Doppler shift determination for these frequency modulated radars.
The above mentioned radars are called operational radars. Beside these radars, there are instrumentation radars, which are capable of producing RF image of objects. The instrumentation radars usually have very high pulse repetition rates and short pulse durations in order to capture the RF images of moving objects and to suppress background contaminations. They are short range in nature, and not for the operational use. The instrumentation radars are only deployed in test ranges.
In light of the above, there is a need in the art for a radar system which is capable of determining both range information and Doppler shifts without stringent interpulse requirement and range ambiguities. Furthermore, the new radar systems are capable of revealing the RF images of objects during field operations.